It is known to carry out hammering of the inner surface of a steam generator tube using particles suspended in a gas travelling at high speed, in order to produce a compression of the inner wall of the tube, especially in the regions of this tube which are liable to be affected by stress corrosion.
A process of this kind and a device for implementing are disclosed in applicants' French Pat. Nos. 2,572,965 and 2,584,320.
This process for producing compression may be applied in particular to the steam generator tubes of pressurized-water nuclear reactors, whose inner diameter is close to 0.020 m. The size of the particles employed for the hammering, the mass flow rate of these particles, and the speed of the carrier gas must be set at accurate values, in order to ensure high effectiveness of the operation of producing compression of the sensitive regions of the inner surface of the tube.
This operation is performed inside the water box of the steam generator, under remote control, by the use of a device comprising an outer housing of tubular shape comprising, at one of its ends, a bearing face which is intended to come into contact with the entry face of the tube plate, in succession around the end of each of the tubes to be treated, which are flush with this entry face. A hammering tool, comprising a nozzle for projecting particles suspended in the carrier gas, is fastened to the end of a flexible sheath and is mounted for movement in the axial direction, in relation to the outer housing. In order to perform the hammering of a tube, the outer housing is placed with its end face in contact with the entry face of the tube plate, around the tube to be treated. The hammering tool is introduced into the tube and is moved inside this tube as far as the region to be treated.
The treatment is then carried out by projection of the particles by means of the carrier gas, the projection nozzle being moved at a low speed in the region to be treated.
The hammering operation must be performed in succession on a very large number of tubes, since steam generators of pressurized-water nuclear reactors contain several thousand tubes. Moreover, these tubes are not accessible during the treatment for producing compression, or after such treatment, because of their radioactivity. The effectiveness of the treatment for producing compression cannot therefore be verified using simple means and a direct method.
One prior art method for measuring the effectiveness of a particle hammering treatment, such as microbead blasting or shot blasting, consists in using a specimen of a standardized type known by the name of Almen, on which the particle hammering operation is performed. French Standard NF L 06,832 of December 1979 accurately defines the test method and the parameters defining the effectiveness of hammering. In particular, the covering of this hammering is determined, and is defined as the ratio of the imprinted area to the area to be hammered or shot-blasted.
A known method for measuring the effectiveness of microbead blasting of the inner surface of a tube, i.e., the hammering of this area with fine beads, consists in carrying out the treatment on a tube of the same diameter and of the same thickness as the tubes to be treated. After the treatment has been performed on the test tube, a slit is made in the surface of this tube, along a generatrix, and then the tube diameter is measured after the slit has been cut. The relative change in the tube diameter is calculated, after the slit has been cut. This value is representative of the effectiveness of the hammering performed on its inner surface. This method has the disadvantage of being time-consuming and of requiring many handling operations in order to produce one measurement of the effectiveness of hammering.
In the case of the treatment of a steam generator of a pressurized-water nuclear reactor, the prior art control methods which are described above can be employed only at regular intervals, after the treatment of a certain number of tubes and outside the water box of the steam generator.
When the control is performed on Almen specimens, during the treatment of producing compression by hammering the tubes of a steam generator, a control is generally carried out on a specimen at regular intervals, after the hammering of a batch of several hundred tubes, for example three hundred tubes, has been performed. To do this, the tool must be withdrawn from the water box of the steam generator, and hence the tube treatment operation must be stopped.
Under the environmental conditions prevailing in the vicinity of the steam generator and, in particular, because of the existence of some radioactivity of the elements employed, certain precautions must be taken to protect the personnel working on the site.
Under these conditions, the time devoted to all the control operations represents approximately 10% of the total time devoted to the operation of producing compression by hammering.
Furthermore, if the control detects a shortcoming in the treatment, the whole batch of tubes whose hammering has been carried out since the last control must be retreated. It is not possible, in fact, to determine from which tube onwards the hammering treatment has turned out to be defective.
No device is known to date which would make it possible to control the effectiveness of an operation of hammering the inner surface of a steam generator tube and to provide information immediately after the hammering of the tube and without having to interrupt the hammering operation taking place.